1. Field of Invention
The present invention generally relates to flextensional piezoelectric transformers and actuators, and more particularly, to a method of manufacturing a flextensional multi-layer piezoelectric transformer/actuator by cofiring the ceramic composition with the electrode forming metallization.
2. Description of the Prior Art
Wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions or in charging devices of copier machines which require high voltage. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
Materials exhibiting piezoelectric and electrostrictive properties develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
The ceramic bodies employed in prior piezoelectric transformers take various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG. 1. This type of piezoelectric transformer is commonly referred to as a "Rosen-type" piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab 10 which is appreciably longer than it is wide and substantially wider than thick. As shown in FIG. 1, a piezoelectric body 10 is employed having some portions polarized differently from others. In the case of FIG. 1, the piezoelectric body 10 is in the form of a flat slab which is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab 10, the portion 12 to the right of the center of the slab is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case the remainder of the slab is actually divided into two portions, one portion 14 being polarized transversely in one direction, and the remainder of the left half of the slab, the portion 16 also being polarized transversely but in the direction opposite to the direction of polarization in the portion 14.
In order that electrical voltages may be related to mechanical stress in the slab 10, electrodes are provided. If desired, there may be a common electrode 18, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion 14 of the slab 10, there is an electrode 20 opposite the common electrode 18. For relating voltages to stress generated in the longitudinal direction of the slab 10, there is a secondary or high-voltage electrode 22 cooperating with the common electrode 18. The electrode 22 is shown as connected to a terminal 24 of an output load 26 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 18 and 20 is stepped up to a high voltage between the electrodes 18 and 22 for supplying the load 26 at a much higher voltage than that applied between the electrodes 18 and 20.
A problem with prior piezoelectric transformers is that they are difficult to manufacture because individual ceramic elements must be "poled" at least twice each, and the direction of the poles must be different from each other.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because it is necessary to apply electrodes not only to the major faces of the ceramic element, but also to at least one of the minor faces of the ceramic element.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because, in order to electrically connect the transformer to an electric circuit, it is necessary to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) to electrodes on the major faces of the ceramic element as well as on at least one minor face of the ceramic element.
Another problem with prior piezoelectric transformers is that the voltage output of the device is limited by the ability of the ceramic element to undergo deformation without cracking or structurally failing. It is therefore desirable to provide a piezoelectric transformer which is adapted to deform under high voltage conditions without damaging the ceramic element of the device.
Piezoelectric and electrostrictive devices (generally called "electroactive" devices herein) are also commonly used as drivers, or "actuators," due to their propensity to deform under applied electric fields. When used as an actuator, it is frequently desirable that the electroactive device be constructed so as to generate relatively large deformations and/or forces from the electrical input. Prior electroactive devices include flextensional transducers which are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. By coupling two or more electroactive devices, the deformation of one "actuator" can cause the deformation of the adjacent coupled actuator.
Another type of transformer, which is disclosed in co-pending patent application Ser. No. 08/864,029 takes advantage of both the electrical properties of electroactive devices as well as the mechanical "actuator" properties. As disclosed in my copending patent application, a piezoelectric transformer can be made by mechanically bonding electroactive devices to each other such that an input voltage is transformed into mechanical movement, which is translated through the mechanical bond to the adjacent electroactive device, which generates an output voltage.
One embodiment of the type of piezoelectric transformer disclosed in my co-pending patent application is illustrated in FIG. 2. This transformer 1 is manufactured by stacking two ceramic wafers 30 and 48 between three preferably metallic layers 36, 42 and 54, bonding them together with four adhesive layers 34, 40, 44 and 52, and simultaneously heating the stack to a temperature above the melting point of the adhesive materials, such as LaRC-SI.TM. developed by NASA Langley Research Center. The adhesive used is a very strong adhesive and has a coefficient of thermal contraction which is greater than that of most ceramics (and, in particular, is greater than that of the materials of the two ceramic wafers 30 and 48). The adhesive is used to apply a bond between the respective metallic layers 36, 42 and 54 and the ceramic wafers 30 and 46 and the bond is sufficient to transfer longitudinal stresses between adjacent layers of the transformer 1.
After the entire stack of laminate layers have been heated to a temperature above the melting point of the adhesive materials, the entire stack of laminate layers is then permitted to cool to ambient temperature. As the temperature of the laminate layers falls below the melting temperature of the adhesive materials, the four adhesive layers 34, 40, 44 and 52 solidify, bonding them to the adjacent metallic layers 36, 42 and 54. During the cooling process the ceramic wafers 30 and 42 become compressively stressed along their longitudinal axes due to the relatively higher coefficients of thermal contraction of the materials of construction of the metallic layers 36, 42 and 54. By compressive stressing the two ceramic members 30 and 42, the ceramic members 30 and 42 are less susceptible to damage (i.e. cracking and breaking).
A piezoelectric transformer constructed in accordance with the preceding description comprises a pair of piezoelectric ceramic wafers 30, and 42 which are intimately bonded to each other (albeit separated by laminated adhesive 34, 40, 44 and 52 and metallic layers 36, 42 and 54) along one of each of their major faces. The metallic layers 36, 42 and 54 and the four adhesive layers 34, 40, 44 and 52 are longer than the two ceramic wafers 30 and 42 and, accordingly, protrude beyond the ends of the ceramic members 30 and 42. Electric terminals 56, 58 and 60 are connected (e.g. by wire and solder, or other common means) to an exposed surface of the metallic layers 36, 42 and 54 respectively.
Referring again to FIG. 2: When a primary (i.e. input) voltage V1 is applied across terminals 58 and 60 connected to the electrodes 32 and 38 of the first ceramic wafer 30, the first ceramic wafer 30 will piezoelectrically generate an extensional stress commensurate with the magnitude of the input voltage V1, the piezoelectric properties of the wafer 30 material, the size and geometry of the wafer 30 material, and the elasticity of the other materials of the other laminate layers (i.e. the ceramic wafer 48, the three pre-stress layers 36, 42 and 54, and the four adhesive layers 34, 40, 44 and 52) which are bonded to the first wafer 30. The extensional stress which is generated by the input voltage Vi causes the first ceramic wafer 30 to be longitudinally strained, (for example as indicated by arrow 64).
Because the first ceramic wafer 30 is securely bonded to the second ceramic wafer 48 (i.e. by adhesive layers 40 and 44), any longitudinal strain 64 of the first ceramic wafer 30 will result in a longitudinal strain (of the same magnitude and direction) in the second ceramic wafer 48 (as indicated by arrow 65). The longitudinal strain 65 of the second piezoelectric ceramic wafer 48 generates a voltage potential V2 across the two electroplated surfaces 46 and 50 of the second ceramic wafer 48. The electric terminals 58 and 56 may be electrically connected to corresponding electroplated surfaces 46 and 50 of the second ceramic wafer 48. The magnitude of the piezoelectrically generated voltage V2 between the two electrodes 46 and 50 of the second ceramic wafer 48 depends upon the piezoelectric properties of the wafer 48 material, the size, geometry and poling of the wafer 48 material.
Thus, by applying a first voltage V1 across the electroplated 32 and 38 major surfaces of the first ceramic wafer 30, the first ceramic wafer 30 is caused to longitudinally strain 64, which, in turn, causes the second ceramic wafer 48 to longitudinally strain 65 a like amount, which, in turn produces a second voltage potential V2 between the electroplated 46 and 50 major surfaces of the second ceramic wafer 48.
The ratio of the first voltage V1 to the second voltage V2 is a function of the piezoelectric properties of the wafer 30s and 48, the size and geometry of the wafers 30 and 48 material, the elasticity of the other materials of the other laminate layers (i.e. the ceramic wafers 30 and 48, the three pre-stress layers 36, 42 and 54, and the four adhesive layers 34, 40, 44 and 52), and the poling characteristics of the two ceramic wafers 30 and 48.
In the one embodiment of the invention disclosed in my co-pending patent application, the facing electroplated surfaces 38 and 46 of the first ceramic wafer 30 and second ceramic wafer 48, respectively, are electrically connected to a common electric terminal 58. In alternative embodiments of the transformer (not shown) the corresponding facing electroplated surfaces 38 and 46 of two ceramic wafers 30 and 48 are electrically insulated from each other, (for example by a dielectric adhesive layer), and connected to corresponding terminals. In this modified embodiment of the transformer the two piezoelectric ceramic wafers 30 and 48 are completely electrically isolated from each other. A transformer constructed in accordance with this modification of the invention may be used in an electric circuit to electrically protect electrical components "downstream" from the transformer from damage from high current discontinuities "upstream" of the transformer.
Although, this type of piezoelectric transformer is simpler to manufacture than prior art Rosen transformers, it is still somewhat difficult to manufacture because the necessity of adhering electrodes between the ceramic wafers and to the major faces of the ceramic wafers.
Another problem with such methods of manufacturing piezoelectric transformers is that it is difficult to maintain complete electrical contact over the entire major face of the ceramic wafer, because of the presence of multiple adhesive layers between the ceramic and metallic layers.
Another problem with piezoelectric transformers manufactured by such a process is that the adhesive bond between the metallic layer and ceramic layer may not be uniform, and the adhesive may delaminate or detach due to deformation of the ceramic layer.
Another problem is that the presence of multiple adhesive layers between the metallic and ceramic layers makes miniaturization of piezoelectric transformers more difficult using such methods of manufacturing.
Another problem with piezoelectric transformers manufactured by such a process is that the multiple adhesive and metallic layers between ceramic layers dampen the motion of the first ceramic layer and limit the translation of motion from the first ceramic layer to the adjacent ceramic layer.